/Engine/Source/Runtime/MeshConversion/Public/MeshDescriptionBuilder.h
Append a triangle to the mesh with the given PolygonGroup ID
[FTriangleID](API\Runtime\MeshDescription\FTriangleID) AppendTriangle ( const [FVertexID](API\Runtime\MeshDescription\FVertexID) * Triangle, const [FPolygonGroupID](API\Runtime\MeshDescription\FPolygonGroupID) & PolygonGroup )